Romeo and Juliet: Balcony Scene Glossary (2.2)

She speaks, yet she says nothing; what of that?
Her eye discourses, I will answer it.
(2.2.13-14)

i.e., She speaks, yet her lips are not moving; what of that? Her eyes speak, and I will answer them.

Henry Theodore Tuckerman wrote an excellent survey of Shakespeare's eye-language:

Othello, when first awakened to jealousy, in order to satisfy his doubts, exclaims to Desdemona, "let me see your eyes!" Alas! That he did not credit their truthful expression! Fear, too, is strongly evinced by the same wondrous organs. In the awful hints the Ghost gives Hamlet of "that undiscover’d country," among the effects prophesied from a more full revelation, is to make his "eyes like stars start from their spheres."

In some eyes, the bard bids us behold "a lurking devil," in others "love’s richest book," – in the poet’s "a fine phrensy;" and, be it remembered, it was upon the eyes that Puck was ordered to squeeze the little purple flower. Perdita with her fine imagination could find no better similitude for "violets dim" than "the lids of Juno’s eyes." Prospero exultingly declares, when Ferdinand and Miranda meet, "at the first glance, they have changed eyes."...The prominent part this miraculous little globe performs in love is indicated by Romeo in Capulet’s garden:
She speaks, yet she says nothing; what of that?
Her eye discourses, I will answer it.
(The Optimist 169)

Tuckerman's book, written in 1850, is now in the public domain and can be downloaded for free on Google books.